1. Field of the Invention
The present invention relates to quantizing and dequantizing circuitry for an image data companding device incorporated in a digital electronic still camera or similar imaging apparatus. The companding device built in a digital electronic camera, for example, selectively compresses image data representative of a scene picked up by the camera and stores the compressed data in a recording medium or reads them out of the medium and expands them.
2. Description of the Related Art
A camera of the kind described digitizes an image signal representative of a shot to generate corresponding digital image data. The digital image data are compressed to less than a desired amount of data by bidimensional orthogonal transform coding or similar coding scheme and then written to an image data recording medium, e.g., a semiconductor memory or a magnetic or optical disk. It has been customary with a bidimensional orthogonal transform coding to divide a single frame of image data into a predetermined number of blocks each having a predetermined size and transform the individual blocks of image data to data lying in the frequency domain, i.e., transform coefficients. The transform coefficients are each quantized by being divided by a quantization coefficient which matches the characteristic of the shot or picture. Specifically, a plurality of quantization coefficients are listed in a look-up table. A CPU (Central Processing Unit) or similar computing device selectively reads the quantization coefficients out of the look-up table and divides the transform coefficients thereby. The quantized transform coefficients are compressed by run-length coding and Huffman coding and then written to a recording medium. This kind of transform coding is successful in using the limited capacity of the recording medium efficiently. For details of compression coding including bidimensional orthogonal transform, a reference may be made to U.S. Pat. No. 5,184,229 to Saito et al. assigned to the same assignee as the present application.
The image data compressed by orthogonal transform coding may be read out of the recording medium and displayed on a monitor or similar display. In this case, the image data read out of the recording medium are decoded by Huffman decoding and then multiplied by a quantization coefficient used for quantization, i.e., dequantized to restore transform coefficients. The data in the frequency domain are restored by inverse orthogonal transform. As a result, the original image data are reconstructed and fed to a monitor.
The conventional quantizing and dequantizing procedure described above has some problems left unsolved, as follows. Since quantization and dequantization are executed by a CPU or similar computing device using software, they are time-consuming. Particularly, in the event of quantization, dividing orthogonal transform coefficients by a quantization coefficient needs a substantial period of time. To eliminate this problem, only the reciprocal of the quantization coefficient may be calculated by the CPU and downloaded to a RAM (Random Access Memory) or similar storage. Then, a multiplier can multiply orthogonal transform coefficients by the coefficient read out of the storage, thereby reducing the quantizing time. However, assuming a reciprocal derived from a quantization coefficient for 8-bit data, it is required to have accuracy higher than eight bits, e.g., sixteen bits in order to guarantee accurate computation. As a result, a substantial period of time is necessary for the reciprocal to be downloaded from the CPU to the RAM. Therefore, although a single circuit effecting both the quantization and the dequantization with a single multiplier and a RAM may be contemplated, the period of time for computing the. reciprocal of the quantization coefficient and the period of time for downloading it from the CPU to the RAM make it difficult to implement such a common circuit scheme.